This might bring me some ridicule, but if it was my gun, I think I might try an appropriately colored crayon. I feel that the wax would adhere to the bare metal of the scratch, while easily being able to be wiped of off the area where the parkerizing is intact.
I realize that this would be a non-permanent solution, but it would be low cost/impact/risk.
I realize that this would be a non-permanent solution, but it would be low cost/impact/risk.